In conventional wireless communication systems, the channel capacity and reliability can be increased by using antenna diversity techniques, wherein multiple antennas are used to provide multiple channels that can mitigate Rayleigh fading effects. For example, multiple channel communications can be used in mobile communication systems, and in satellite communications systems where a single user transmits a signal through several satellite transponders to a common base-station. Several conventional approaches allow the signals received from multiple transmission paths (i.e., multiple channels) to be combined at the receiver. Some examples of these are: selection diversity, wherein the received channel with the largest signal-to-noise ratio is selected, and the remaining channels are discarded; equal gain combining, wherein the receiver independently estimates and removes channel parameters with respect to each received channel, and then equally combines the detected channel symbols; and maximum ratio combining (MRC), wherein the receiver independently estimates and removes channel parameters with respect to each received channel, and then combines the channel symbols proportionally by the strength of the signals received on the respective channels. In these and other conventional approaches, channel parameters such as phase and time delay are estimated before the received signals are combined.
FIG. 14 diagrammatically illustrates a Single Input Multiple Output (SIMO) wireless communications system according to the prior art. In the exemplary system of FIG. 14, typical of satellite communications systems that are used for both communications and user geo-location, a ground station receiver 140 cooperates with a common (i.e., single) transmitter, and several transponders shown generally at 141. The common transmitter transmits a signal to the transponders at 141, which in turn transmit the signal to the receiver 140. (For clarity and conciseness of exposition, the example of FIG. 14 shows only two transponders and their associated channels.) The receiver 140 includes two antennas that receive two respective versions of the transmitted information message, one associated with each channel. The receiver 140 also includes channel estimators that estimate unknown parameters associated with the channels, for example, unknown time delay parameters and unknown phase parameters. Compensators at 142 appropriately compensate for the estimated channel parameters. The estimation of channel parameters, and the associated compensation, is performed before the signals are combined at 143 and decoded at 144. In FIG. 14 (and in FIG. 15 described hereinbelow), the “h” characters represent characteristics of wireless communication links (between respective pairs of communicating antennas) in the respective transmission channels, and the “n” characters represent noise in the associated channels.
At low signal-to-noise ratios, the aforementioned unknown channel parameters can be difficult to estimate, which can adversely impact the aforementioned signal combing and decoding. It is therefore desirable to provide for improvements in wireless communications at low signal-to-noise ratios.